In at least one known x-ray imaging system configuration, commonly known as a computed tomography (CT) system, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane, generally referred to as the "imaging plane", of a Cartesian coordinate system. The x-ray beam passes through the object being imaged, such as a patient. After being attenuated by the object, the beam impinges upon an array of radiation detectors. Intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
The x-ray source typically includes a cathode and an anode that emits x-rays during operation. In one known x-ray source, the cathode includes a tungsten thermionic emitting source and focusing surfaces. A filament heats the cathode to an operating temperature and, upon application of a potential across the cathode and anode, the cathode produces thermionically emitted electrons which traverse a vacuum gap to the anode and impact the anode at a focal spot, thereby generating x-rays.
In the known x-ray source described above, the anode rotates at a high speed so that heat generated at the focal spot is distributed over the anode surface. Distributing the heat over the anode surface is particularly important in diagnostic x-ray applications, such as cardiac interventional procedures, that require high focal spot intensities.
In the known x-ray source, the anode is rotated by an induction motor having a cylindrical rotor coupled to a cantilevered axle that supports a disc-shaped x-ray tube anode. The rotor is supported by ball bearings in the cantilevered axle, sometimes referred to as a rotor cage. The rotor is often required to rotate at speeds reaching 17,000 revolutions per minute (rpm). The ball bearings thus have to satisfy exceptional demands.
As the ball bearings and rotor wear, the rotation of the anode may be adversely affected. For example, anode revolutions per minute may decrease or the anode may not rotate concentrically. Eventually, if the worn components are not repaired or replaced, the x-ray source will fail.
Failure of an x-ray source, particularly if such failure occurs while performing a patient scan, is highly undesirable. For example, if an x-ray source fails during a scan, the patient dose may exceed the intended level. Therefore, it would be desirable to provide apparatus and methods for quickly and easily detecting the onset of a potential x-ray source failure. It also would be desirable to provide a system which estimates x-ray source life expectancy so that unexpected x-ray source failures can be avoided.